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How do you categorize non-functional requirements?

Posted by Adrian M.

Article Rating // 8010 Views // 0 Additional Answers & Comments

Categories: Requirements Analysis (BABOK KA), Elicitation (BABOK KA)


There are many different ways to categorize non-functional requirements depending on the type of system, project, organization, or preference.

Here are some common ways of categorizing requirements:

1. Qualities vs. Constraints

Qualities of a System: these are characteristics or properties of a system that the stakeholders care about. These are commonly sub-categorized as: 

  • Run-time Qualities: A subset of non-functional requirements, these are generally qualities which describe how well the functional requirements behave/are perceived when the system is being used:
    • usability,
    • configurability,
    • supportability,
    • correctness,
    • availability and reliability,
    • quality of service requirements: performance, response time, latency, etc.
    • security, 
    • fault tolerance, 
    • scalability
  • Development-time Qualities: A subset of non-functional requirements, these are qualities of the system, architecture, documentation, and design which have an impact on the effort, ease, and cost of maintaining and changing the system over time: 
    • localizability—ability to make adaptations due to regional differences
    • modifiability or extensibility—ability to add (unspecified) future functionality
    • evolvability—ability to change over time and to integrate and use new technologies
    • composability—ability to compose systems from components
    • reusability—ability to (re)use in future systems

Constraints of a System: these are characteristics of the pre-existing environment within which the new system or solution must operate, such as: legal and regulatory constraints, available development frameworks, target hardware, knowledge and skill of the development team, time and budget, etc.

2. Execution vs. Evolution

Execution requirements: Qualities a system must have which are observable at run time: security, usability, etc.

Evolution requirements: Qualities a system must have which address the nature and structure of the system and which permit a system to be evolved in the future: scalability, extensibility, etc.

3. Many categories: most organizations simply maintain a long list of non-functional requirement categories including but not limited to:

  • Accessibility
  • Audit and control
  • Availability
  • Certification
  • Dependency on other parties
  • Documentation
  • Efficiency (resource consumption for given load)
  • Effectiveness (resulting performance in relation to effort)
  • Escrow
  • Extensibility (adding features, and carry-forward of customizations at next major version upgrade)
  • Infrastructure
  • Legal and licensing issues
  • Maintainability
  • Operating constraints
  • Performance / Response time
  • Platform compatibility
  • Price
  • Quality (e.g. Faults Discovered, Faults Delivered, etc.)
  • Reliability (e.g. Mean Time Between Failures)
  • Resilience
  • Resource constraints (processor speed, memory, disk space, network bandwidth etc. )
  • Robustness
  • Scalability
  • Software, tools, standards etc.
  • Stability
  • Supportability
  • Usability and User Experience

How do you categorize non-functional requirements?



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